2nd Nordic Cosmology Meeting

Session

Contributed talks

12 Mar 2025, 14:45

E204 (University of Helsinki)

4. Symmetry-breaking inflation in non-minimal metric-affine gravity

Antonio Racioppi (National Institute of Chemical Physics and Biophysics (EE))

12/03/2025, 14:45

Talk

We study symmetry-breaking inflation within the framework of metric-affine gravity. By introducing a non-minimal coupling, $\beta(\phi)\tilde{\cal R}$, between the Holst invariant and the inflaton, both small-field and large-field inflationary predictions can be brought into agreement with the latest observational constraints. Remarkably, even for sub-Planckian vacuum expectation values,...

15. Axion inflation with non-Abelian gauge fields

Oksana Iarygina (Nordita)

12/03/2025, 15:05

Talk

Currently, the search for primordial gravitational waves is largely focused on detecting the parity-odd polarization pattern in the Cosmic Microwave Background—the B-modes. Accurately interpreting B-mode measurements depends heavily on understanding their production mechanisms. A particularly compelling scenario involves gravitational wave generation through the interaction of axion with gauge...

37. New early dark energy and its equation of state

Aleksandr Chatrchyan (Nordita)

12/03/2025, 16:00

Talk

The Hubble Tension is the discrepancy between the measured value of the Hubble parameter $H_0$ and its $\Lambda$CDM model prediction using CMB data. New Early Dark Energy (NEDE) addresses this tension using a triggered phase transition in the dark sector. In this work we constrain the properties of NEDE using recent CMB and supernovae datasets, as well as the new BAO data from the DESI survey....

16. Quasi-Palatini Gravity and Scalar-Tensor Theories

Sotirios Karamitsos (University of Tartu)

12/03/2025, 16:20

Talk

The Palatini formulation of gravity has been successfully used to motivate several observationally viable models of inflationary expansion. While the Palatini and metric formulations are equivalent in minimally coupled gravity, non-minimal scalar models are phenomenologically distinct depending on which formulation is used. At the same time, the recent hybrid approach to metric and Palatini...

33. Global portraits of inflation in nonsingular variables

Laur Järv (University of Tartu)

13/03/2025, 12:40

Talk

In the phase space perspective, scalar field slow roll inflation is described by a heteroclinic orbit from a saddle type fixed point to a final attractive point. In many models the saddle point resides in the scalar field asymptotics, thus for a comprehensive view of the dynamics a global phase portrait is necessary. For this task in the literature one encounters dynamical variables that...

26. Cosmological Implications of a New Dark Sector Phase Transition after BBN

Florian Niedermann

13/03/2025, 14:45

Talk

In this talk, I will discuss the phenomenology of a first-order phase transition that occurs after Big Bang nucleosynthesis (BBN) but before recombination in a sector that is secluded from the standard model. Such a rather late phase transition impacts curvature perturbations at a time when some of them have already re-entered the cosmological horizon. In particular, the trigger dynamics of...

12. Holography for phase transitions

Mikel Sánchez Garitaonandia (Ecole Polytechnique)

13/03/2025, 15:05

Talk

First-order phase transitions represent a golden opportunity into uncovering new physics as well as into the QCD phase diagram. These transitions typically proceed through nucleation, expansion and collision of bubbles. As a consequence, the spectrum depends on out-of-equilibrium properties that are challenging to obtain, such as the wall speed. In this talk I will review recent progress in...

35. Thermal nucleation in perturbation theory

Dr Joonas Hirvonen (University of Nottingham)

13/03/2025, 15:25

Talk

Cosmological first-order phase transitions may have generated an observable gravitational wave background, offering a unique probe of beyond-Standard-Model physics. A crucial step in predicting this background is the reliable computation of bubble nucleation rates. In this talk, I will give an overview of recent advancements in perturbative high-temperature nucleation rate calculations. These...

19. Gravitational waves with the Pencil Code

Prof. Axel Brandenburg (Nordita)

13/03/2025, 16:15

Talk

The gravitational wave module in the Pencil Code is a versatile tool for computing the time-dependent three-dimensional strain field from a range of different sources in the early universe ranging from phase-transitional hydrodynamic and magnetohydrodynamic stresses to inflationary ones involving electromagnetic and axion pseudoscalar stresses. The solver utilizes the analytic solution for...

22. Absence of propagating gravitational-wave polarisations in degenerate f(R) models

Adrián Casado-Turrión (Center for Physical Sciences and Technology (FTMC))

13/03/2025, 16:35

Talk

In the context of $f(R)$ gravity, as well as other extended theories of gravity, the correct counting of globally well-defined linear dynamical modes (i.e. gravitational-wave polarisations) has recently drawn a vivid interest. In this talk, we present a consistent approach shedding light on such issues for both so-called degenerate and non-degenerate $f(R)$ models embedded in Minkowski and de...

29. The NANOGrav 15 yr Data Set: Running of the Spectral Index

Rafael Robson Lino dos Santos (NCBJ, Warsaw)

13/03/2025, 16:55

Talk

In this talk I will review the succesful search for Hellings-Downs correlations by the NANOGrav collaboration and the search for new physics with pulsar timing data. In particular, I will show the first model-independent search going beyond the simple power-law fit to characterise the frequency spectrum of the signal observed by the collaboration, allowing for a running of the spectral index....

18. Black holes and gravitational waves from slow phase transitions

Ville Vaskonen (University of Padova)

13/03/2025, 17:15

Talk

Cosmological first-order phase transitions may have played a significant role in the formation of baryon asymmetry and dark matter, potentially leaving an observable gravitational wave signature. In this talk, I will focus on the possibility of primordial black hole formation from slow and strongly supercooled transitions. I will describe how the stochastic nature of the bubble nucleation...

27. Primordial Black Holes from the Running-Mass-Inflation

Mindaugas Karciauskas (Center for Physical Sciences and Technology (FTMC))

14/03/2025, 10:30

Talk

The running-mass inflation model generates the primordial curvature perturbation spectrum that is consistent with observations at the pivot scale but exhibits a very large running at smaller scales. This creates favourable conditions for the formation of primordial black holes. In the current work we analyse a novel mechanism that provides the graceful exit for this model and enables us to...

6. Revisiting butterfy effects for asymptotically Lifshitz black hole

Dr Adrita Chakraborty (AGH University of Krakow)

14/03/2025, 10:50

Talk

We explore connections between two salient chaotic
features, namely Lyapunov exponent and butterfly velocity, for the class of asymptotically Lifshitz black hole background with arbitrary critical exponent by implementing three different holographic approaches, namely,
entanglement wedge method, out of time-ordered correlators (OTOC) and pole-skipping. We present a comparative study where...

25. Evolution of black hole-string networks

Asier Lopez Eiguren (University of the Basque Country)

14/03/2025, 11:40

Talk

Primordial black holes (PBHs), potentially formed in the early universe, may play a crucial role in cosmology, serving as seeds for supermassive black holes and contributing to the formation of merging black hole binaries observed by LIGO/VIRGO. They have also been proposed as candidates for dark matter. Another scenario involves cosmic strings, hypothetical linear defects predicted by various...

10. Networks of Axion Strings - Scaling

Jose Ricardo Correia (University of Helsinki)

14/03/2025, 12:00

Talk

In the post-inflationary scenario, if PQ symmetry breaking occurs, a network of global cosmic strings are expected to form. At the time of the QCD phase transition, the previously scaling network of such defects will collapse by forming domain walls attached to strings. Given that all of the energy density of the string network is left behind into axion waves, in order to improve current...

32. ΛCDM theory of cosmology

Tomi Koivisto (University of Tartu)

14/03/2025, 12:20

Talk

The bulk action of the Yang-Mills theory based on the compact Lorentz group Spin(4)=SU(2)xSU(2) is determined by one parameter c_T. When c_T=1, the theory is right-handed and predicts a spin-2 field with the dynamics of the metric tensor in general relativity in the presence of dust. When c_T=0, the theory is achiral and has no local degrees of freedom. A transition from the latter to the...

45. The Adiabaticity Condition Revisited: Necessary but not Sufficient

Subodh Patil

14/03/2025, 12:40

Talk

We present a simple counterexample that illustrates the fact that a standard expression of the adiabaticity condition -- $\abs \dot\omega/\omega^2\abs \ll 1$, where $\omega$ corresponds to the characteristic frequency of any given field mode -- is a necessary but not sufficient condition for particle production. We further illustrate this via the example of a Mukhanov-Sasaki-like variable...

13. Gravitational Waves and Dark Matter from Scale Symmetry Breaking

Alexandros Karam (NICPB, Tallinn)

Talk

We present an updated analysis of the first-order phase transition associated with symmetry breaking in the early Universe in a classically scale-invariant model extended with a new SU(2) gauge group. Including recent developments in understanding supercooled phase transitions, we compute all of the model's characteristics and significantly constrain the parameter space. We then predict...

2. Invisible Higgs decay from dark matter freeze-in at stronger coupling

Vinicius Oliveira

Talk

We study the Higgs boson decay into dark matter (DM) in the framework of freeze-in at stronger coupling. Even though the Higgs-DM coupling is significant, up to order one, DM does not thermalize due to the Boltzmann suppression of its production at low temperatures. We find that this mechanism leads to observable Higgs decay into invisible final states with the branching fraction of 10% and...